1. Field of the Invention
The present invention concerns a method for determination of movement by an examination subject during the acquisition of magnetic resonance measurement data. Furthermore, the invention concerns a magnetic resonance apparatus that implements such a method.
2. Description of the Prior Art
Magnetic resonance (MR) technology is a known modality with which images of the inside of an examination subject can be generated. For this purpose, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla and more) in an MR apparatus such that nuclear spins in the subject orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to trigger nuclear magnetic resonances, the triggered nuclear magnetic resonances being measured and MR images being reconstructed therefrom. Rapidly-switched magnetic gradient fields are superimposed on the basic magnetic field for spatial coding of the measurement data. The acquired measurement data are digitized and stored as complex number values in a k-space matrix. An MR image can be reconstructed from the k-space matrix populated with these values by means of a multi-dimensional Fourier transformation.
Due to its relatively long measurement time, the MR imaging is movement-sensitive, meaning that movement of the examination subject during the acquisition of the measurement data can contribute to significant limitations in the image quality.
Various methods therefore exist that seek to detect movement of the examination subject and to use the acquired information either for improved reconstruction of the image, or for a prospective adaptation of the measurement system for the subsequent acquisition of measurement data.
Somewhat elaborate methods of this type utilize external markers and structural systems with which movement can be detected with optical means three-dimensionally in space and taken into account. Such methods require additional hardware and thus incur a high cost expenditure, so that such methods are not typically used.
Moreover, methods are known in which a special design of the measurement sequence enables the movement detection. Given the acquisition of the measurement data in the PROPELLER technique (as it described, for example, in the document by J. G. Pipe, “Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) MRI; Application to Motion Correction”, ISMRM 1999, Abstract Nr. 242), a k-space matrix is scanned in segments, with the individual k-space segments being rotated relative to one another so that a central k-space region is scanned with each k-space segment. The sub-sampling of the central k-space region enables a movement that occurs between the scanning of the individual k-space segments to be detected and to be taken into account in the image reconstruction.
Another method is used in functional MR imaging (fMRI″) and is known under the name PACE (for “prospective acquisition correction”). Sequential, complete multi-slice single-shot EPI (for “echo planar imaging”) data sets of the brain are acquired while various stimuli are presented to a subject. In order to be able to compare the repeatedly acquired data sets with one another, it is necessary that the three-dimensional data sets be positioned and aligned identically relative to one another. In the PACE method a data set is evaluated in the acquisition of the measurement data to allow the acquisition of the following data set to ensue dependent on any change in the position of the examination subject that may have occurred.
The above-described methods are tailored to the special design of the employed measurement sequence, and typically cannot be transferred to other measurement sequences.
A different method used in many cases for detection and/or for correction of movements occurring during the acquisition of the measurement data is the use of what are known as navigator signals (also called navigator echoes).
In this type of acquisition, additional data (known as navigator signals) are acquired in addition to the actual measurement data with which the k-space matrix corresponding to the image to be produced is populated. These navigator signals allow a movement of the examination subject that occurs during the acquisition of the measurement data to be detected and allow this to be taken into account if applicable in the reconstruction of the MR image or images, such that movement artifacts occur to a lesser extent.
A small region of the k-space matrix (for example one k-space line or a small central section of the k-space matrix) is typically scanned by the navigator signal. Movement that may possibly have occurred between the scanning of the two navigator signals can be detected and/or taken into account in the image reconstruction by a comparison of the k-space values scanned by the navigator signal with regard to their amplitude and phase position.
Depending on the complexity of the navigator signal, the measurement duration of a measurement sequence sometimes increases significantly due to the acquisition of such navigator signals.
A need therefore exists to improve methods of the type wherein a possible movement of an examination subject is detected with the aid of navigator signals.